A semiconductor device structure having a nitride semiconductor layer is a known structure for a semiconductor device. An aluminum gallium nitride (AlGaN) layer, a gallium nitride (GaN) layer, and the like may be employed as this nitride semiconductor layer. A semiconductor device provided with such a nitride semiconductor layer is considered to have promise as a switching element used in a switching power supply, an inverter circuit, and so forth.
When the switching element is used in the switching power supply, the inverter circuit, and so forth, there may be a case where a high voltage is applied to the switching element in an off state and the switching element breaks down due to a phenomenon known as avalanche breakdown. Thus, the switching element is required to have a certain level of avalanche tolerance.
However, if the semiconductor device provided with the nitride semiconductor layer is required to have avalanche tolerance with which the semiconductor device can withstand a high voltage, undesirable results of such a design include an excessively large margin of an element breakdown voltage and an increase in an element area.